Offshore platform for ice conditions



Oct. 24, 1967 K. A. BLENKARN ET AL 3,343,332

OFFSHORE PLATFORM FOR ICE CONDITIONS Filed April 21, 1965 [Illlllll GEORGE C. HOWARD KENNETH A. BLENKARN INVENTORS.

ATTORNEY.

United States. Patent 3,348,382 OFFSHORE PLATFORM FOR ICE CONDITIONS Kenneth A. Blenkarn and George C. Howard, Tulsa,

Okla., assignors to Pan American Petroleum Corporation, Tulsa, Okla., a corporation of Delaware Filed Apr. 21, 1965, Ser. No. 449,692 1 Claim. (Cl. 61-46) ABSTRACT OF THE DISCLOSURE A marine platform especially arranged to minimize impact from ice fioes or the like, includes a set of vertical girders or columns strongly cross braced from the bottom of the water to around to 20 feet below low water line. The structure is also characterized by no cross bracing up to about 10 feet above the high water line, above which cross bracing continues to the operating platform. Space between adjacent vertical members in the platform preferably lies between about three to about eight diameters of the vertical members.

In recent years marine drilling for oil has progressed in many areas. One difficulty experienced in northern waters which is not found in more temperate climates, is the presence during the colder months of ice fioes of tremendous mass. These may batter any marine structure and have been known to destroy completely marine platforms built in accordance with ideas pertinent to land structures.

We have found a type of rigid support for a marine platform which furnishes minimal possibility of damage from impact of ice fioes, or the like, while offering maximum rigidity and minimum cost for such a structure. Accordingly, this invention provides an apparatus, or structure, for support of a platform, the structure withstanding floating ice conditions.

The attached drawing forms a part of the specification and should be read in connection therewith. The sole figure shows, in diagrammatic view, a cross section of the structure.

Basically, this structure consists of a plurality of vertical girders or legs 11 (usually pipes), the length of which is great compared with the diameter, for example such vertical girders 11 being made of extra heavy pipe ten to twenty feet in diameter and at times well over a hundred feet long. The lower end of each girder 11 is firmly aflixed in the ground 12 below the body of water 13 in which the structure is erected. This may be effected after the girders 11 have been vertically positioned on the sea floor by driving piling 14 (shown diagrammatically) of smaller size than the inner diameter of each girder 11 through the bottom of the girders 11 and into the ground 12 and welding these girders 11 to the piling 14, or the like. There are three or more (preferably four) vertical girders or legs 11, arranged in a regular pattern, and unless very unusual conditions prevail, the spacing between adjacent girders or legs is substantially constant.

The girders are very rigidly cross-braced by members 15 and 16. It is a feature of this design that this crossbracing extends from substantially the level off the ground 12 up to, but only up to, a distance D below extreme mean low water level 17. Usually the cross-bracing members 15 and 16 will have been aflixed, for example by welding or other framing techniques, to the substantially vertical girders 11 before the structure has been moved into place. It is possible, for example, to make these members of hollow pipe of the order of thirty-inch diameter pipe welded at the ends to the girders 11 and associated cross-braces. This minimizes horizontal deflection of the girder 11 which will occur when an ice floe 18, or the like, impacts on the girders. The structure preferably will have equivalent cross-bracing between alternate, non-adjacent girders 11, though this may not always be necessary.

It is essential, for successful operation, that the entire structures supporting the top platform 19 (shown in very rough diagrammatic form) offer maximum strength and minimum impact area for the floe 18. Cross-bracing of both the submarine structure from the depth D essentially to the total depth of the water above ground 12, and of the platform 19 above a height H (to be discussed below) must be provided. The other essential is that all crossbracing be eliminated throughout the vertical height of the structure represented by D and H, which are essentially the dimensions required for clearance from any floe 18.

Our studies have shown that in northern regions, the maximum mass of ice ordinarily encountered is not in an iceberg but is in an ice fioe, i.e. in which horizontal area is large compared to vertical thickness. It is the impact of such a floe with the marine platform that presents maximum possibility of damage. While the thickness of the ice floe may vary considerably, we have found that the maximum depth d of ice which we have encountered in the floe has been of the order of ten feet and has never, in our experience, exceeded nineteen feet. Minimum horizontal force is imparted to the vertical girders 11 if there is no cross-bracing (such as members 15 and 16) at a depth D below the water surface which exceeds the maximum depth d of the floe. Accordingly, we prefer to have the cross-bracing in the submarine part of this structure be limited to at least ten feet, and preferably twenty feet, below the lowest water level encountered in the region.

The spacing between the vertical girders in the region in which there is no cross-bracing is of critical importance. We have observed that, in general, when an ice floe under the action of water current and wind impinges on the vertical girders of such a structure, the floe is destroyed at the area of contact by a combination of shearing, crumbling, etc. If the vertical girders are too closely spaced together, the complete volume of ice between the outermost portions of two adjacent girders is involved in this action, with a maximum horizontal force being imparted to the girders. As the girders are spaced more widely apart, the horizontal force involved becomes less. The shearing and crushing taking place at each girder (except possibly one) is less because the floe has been weakened by the roughly parallel channel in the floe being formed at the adjacent girder. However, when the spacing between the girders becomes large, the horizontal force against the structure as the floe is crushed increases again, since the channel in the ice being formed at one girder is too far away to affect the strength of the ice at the adjacent girder.

For these reasons we definitely prefer that the spacing between the girders be a minimum of a least 3 and a maximum of not over about 8 times the outer girder diameter. If the vertical girders are not all of uniform diameter, we prefer that the spacing between adjacent girders be a minimum of about 3 up to a maximum of about 8 times the average girder diameter.

With reference to the attached figure, the ratio of dimension B to dimension A, i.e. spacing between girders to girder diameter at the water level, should be in the range of about 3 to about 8.

Since the density of ice is near to but less than that of water, it would be expected that little vertical clearance H would be required between high water level and the cross-bracing of the platform 19 above it. However, it is our experience that there are ice pressure ridges 20, or

Patented Oct. 24, 1967 local high places, found on fioes which extend considerably above the average level of the floe. It is our experience that the maximum height of such pressure ridges It may be as much as nine feet above the water level. We find that marine floes possessing such pressure ridges under low temperature conditions have great mechanical strength and that impinging of even the top of the pressure ridge with the structure can transfer a considerable force to the structure. \Ve, accordingly, design our rigid braced, box type marine structure with such clearance H between high water level and the girder type rigid top platform 19 that at extreme high water level the lowest cross-brace of top platform 19 is at least ten feet but not more than twenty feet above the high water level.

It must be further emphasized that it is quite desirable not to exceed the dimensions given. Thus, for example, it is not desirable to increase the vertical clearance between the lowest members of the top platform 19 and the maximum high water level, nor the clearance D between the maximum elevation of the submarine crossbracing and the extreme low water level at the location of the platform.

Absence of cross-bracing in this region means that the only effect on the platform of the moving ice floe is due to shearing and breaking of the ice floe by the vertical girders 11 in the zone subject to ice contact. Since all the rest of the vertical distance from the ocean floor 12 to the top deck of the marine platform has been rigidly cross-braced, maximum resistance to the horizontal force occasioned by this ice shearing has been provided, and the force is thus transmitted to the piling 14 in the earth 12.

While one particular form of cross-bracing has been shown, it is to be understood that any continuous framing of the girders 11 in the region of interest may be employed, the design criterion being that the structure upon application of horizontal force due to ice impact should fail by overturning about the opposite base of the marine platform rather than by a destruction of the cross-bracing or shear of a girder 11.

Since it is found that the maximum bending effects due to the impact of fioes on this type of structure occur in the part of the girders which are not cross-braced, it is desirable to fill at least the major part of the inner diameter of the girder in this region, and preferably to a depth below this region equal to the length of the first panel of bracing, with a material which will preferably both increase structural strength against bending and the like, and additionally will provide damping against vibration occasioned by the impact of the floating biects. This can be provided, and preferably should be provided, by filling from the level marked 22 up to at least the top of the first panel of cross-bracing above the water level (preferably to the top of the structure) with cementaceous material such as neat cement, concrete, sand and asphalt mixtures, or the like. In a marine structure of the type shown, particularly when employed in the drilling of wells, it is contemplated that the wells will be drilled through the girders 11. That is, such wells will be located entirely within the inner diameters of such girders. The piles which extend through this region in the normal marine structures of this type provide openings through which the wells are then drilled. Accordingly, this cementaceous material should be placed in the annular volume within the girder and around the piles. It is recognized, of course, that the presence of such material near the outside of the inner diameter of the girders is more effective for both structural strength and damping than such material near the center of such a girder. Accordingly, in large diameter girders (of the order of 12 feet outside diameter or greater) it is frequently sufiicient to fill with the cementaceous material only approximately the outer half of the inside radius of the girder.

It will be apparent that the design shown in characterized by its simplicity and economy. It is to be also noted that the design is characterized by proctection chiefly against marine floating objects, rather than horizontal forces due to hurricanes, water currents, or the like. In case of hurricanes this kind of structure, for example, would be designed with the girders 11 extending outward at a considerable angle, i.e. in a truncated structure. If ocean currents were the chief factor, since they would extend essentially from top to bottom of the sea water, such extensive cross-bracing would not be used. The same is true for designs minimizing forces due to wave action which, again, extend to considerable depth below the wave troughs.

We claim:

A marine structure capable of withstanding impact from ice floes or similar floating objects comprising a rigid braced structure containing at least three substantially vertical legs at approximately equal distances apart, the ratio of spacing between adjacent legs at the water level to the average outer diameter of said legs at said level being between about three and about eight,

said structure being provided with continuous cross bracing between adjacent legs extending from at least the bottom of the water to a level at least 10 feet below, but not substantially exceeding 20 feet below low Water level,

said legs extending a substantial distance above the water level,

a second cross-braced structure including a top platform mounted upon and rigidly attached to said legs at a distance of at least 10 feet above but not substantially more than 20 feet above high water level,

said platform including flooring suitable for carrying out marine operations from said platform,

said legs containing no cross bracing between said rigid-braced structure and said rigid-braced top platform, and

a filling of cementaceous material occupying at least the major part of the volume inside each of said legs from the inner diameter towards the center thereof and at least throughout the length of said legs in which there is no cross bracing.

References Cited UNITED STATES PATENTS FOREIGN PATENTS 1958 France.

DAVID J. WILLIAMOWSKY, Primary Examiner.

JACOB SHAPIRO, Examiner, 

